This invention relates to semiconductor devices for controlling light such as optical modulators and nonlinear optical devices.
Conventional semiconductor optical modulators have traditionally made use of the Franz-Keldysh effect to modulate an incident light beam. According to the Franz-Keldysh effect, the band structure of the semiconductor material is shifted by application of an electric field. However, the Franz-Keldysh effect is characterized by small shifts in optical properties such as absorption and index of refraction thereby limiting the modulation to be correspondingly small. In order to achieve the deep modulation required for a practical device it is necessary to use either a high electric field or a device having long optical path length or both. For example, a modulator described by Honda in U.S. Pat. No. 3,791,717 issued Feb. 12, 1974 uses a high electric field (10.sup.5 to 10.sup.6 volts per centimeter) in a semiconductor having a crystal of long optical pathlength (10 microns).
An optical modulator which uses a heterojunction semiconductor device in which optical absorption and reemission are controlled by an electric field is described by Chang et al in U.S. Pat. No. 4,208,667. Chang et al uses a heterojunction superlattice having two different material arrangements alternatively to form a semiconductor heterojunction in which the bottom of the conduction band of a first layer is lower than the conduction band in the second layer, and also the top of the valence band is lower than the corresponding band in the second layer. This device and superlattice allows electrons and holes to be excited by photoabsorption wherein the electrons collect in one layer and holes in the adjacent layer. Charge carriers recombine by making a transition into the adjacent layer with the subsequent emission of light. The rate of recombination is controlled by an electric field applied to the superlattice. It is a property of the Chang et. al. device that the emitted light is of a different frequency from the incident light and that the light is emitted in all directions. In should also be noted that the incident and emitted light beams are not collinear through the devices. As a result, the Chang et al device is impractical as an optical modulator.
Nonlinear optical devices have been made using heterojunction semiconductor materials. These devices are characterized by an operating point determined by optical cavity gain or finesse. These nonlinear optical devices exhibit particular aspects such as bistability, amplification, photonic modulation or the like. A problem with this type of nonlinear optical device is that the operating point is selected by the choice of materials and other design parameters during fabrication of the device. Therefore, the operating point cannot be conveniently controlled at the time the device is in use.